Annealed metal nano-particle decorated nanotubes

ABSTRACT

Disclosed are methods and systems of providing carbon nanotubes decorated with polymer coated metal nanoparticles. Then, annealing the metal coated carbon nanotubes to reduce a quantity of hydrophilic components of the polymer coating.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No.DE-AR0000542 MONITOR-SPHINCS awarded by the Advanced Research ProjectsAgency-Energy. The Government has certain rights in this invention.

TECHNICAL FIELD

Implementations of the present disclosure relate to nanoparticle basedmaterials for gas sensors.

BACKGROUND

Various techniques may be utilized for gas leak detection, with eachapproach having advantages trade-offs. Such techniques include, forexample, catalytic bead sensors, metal-oxide-semiconductor (MOS)sensors, non-dispersive infrared sensors, and infrared laser-basedsensors. Some techniques may be compact and integrated into printedcircuit boards. Depending on materials and techniques used, variousmethodologies may provide different size, power level, expense, or othertradeoffs.

BRIEF DESCRIPTION OF THE DRAWINGS

The described embodiments and the advantages thereof may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings. These drawings in no waylimit any changes in form and detail that may be made to the describedembodiments by one skilled in the art without departing from the spiritand scope of the described embodiments.

FIG. 1 is a diagram of an example nanoparticle based gas sensingmaterial, according to some aspects of the disclosure.

FIG. 2A is a diagram of an example nanoparticle based gas sensingmaterial, according to some aspects of the disclosure.

FIG. 2B is a diagram of an example nanoparticle based gas sensingmaterial, according to some aspects of the disclosure.

FIG. 3 is a diagram showing annealing processes of an examplenanoparticle based sensing material, according to some aspects of thedisclosure.

FIG. 4 is a flow diagram depicting an example method of fabricating ananoparticle based sensing material, according to some aspects of thedisclosure.

FIG. 5 is a flow diagram depicting an example method of fabricating ananoparticle based sensing material, according to some aspects of thedisclosure.

FIG. 6A is a diagram of an example gas sensor using a nanoparticlematerial, according to some aspects of the disclosure.

FIG. 6B is a diagram of an example gas sensor using a nanoparticlematerial, according to some aspects of the disclosure.

FIG. 6C is a diagram of an example gas sensor using a nanoparticlematerial, according to some aspects of the disclosure.

FIG. 7 is a flow diagram depicting an example method of fabricating ananoparticle based sensor, according to some aspects of the disclosure.

DETAILED DESCRIPTION

Natural gas leaks even in low levels, contribute greatly to globalwarming. For Example, methane leaks in an industrial setting mayaccumulate in the atmosphere and produce greenhouse effects. Inaddition, natural gas or methane leaks may also produce hazardousenvironments that may be flammable or produce potential healthconsequences to persons exposed to such environments. Some techniquesfor measuring natural gas or methane leaks including catalytic beadsensors, MOS sensors, non-dispersive infrared, and infrared laser-basedapproaches may be limited in detection limit, size, power consumption,expense, or other limitations.

Compact devices with low power consumption may provide improved sensorcharacteristics for monitoring and reacting to natural gas or methaneleaks. However, some compact and low power gas sensors may have reduceddetection capability in the presence of certain gases. For example,certain sensors for detecting natural gas or methane may not functionproperly in the presence of water vapor. Due to the ubiquitous nature ofwater vapor in normal atmospheric conditions where gas sensors may behelpful, these sensors may have limited lifespans before saturation ofthe sensors with water vapor. The impact of water vapor on such sensorsmay reduce the reliability of the devices.

Described herein are embodiments of metal nanoparticle-decoratednanotubes, methods for producing the same, and sensors incorporating thesame. Certain embodiments relate to a gas sensor that uses a compositioncomprising carbon nanotubes (e.g., single-walled carbon nanotubes, or“SWCNTs”) decorated with metal nanoparticles as a sensing material. Themetal nanoparticles may be coated with a polymer to attach them to theSWCNTs and reduce adsorption of water vapor by the metal nanoparticles.The decorated SWCNTs may then be annealed at a high temperature tocarbonize the polymer coating as well as removing oxygen containingfunctional groups from polymer coating and the SWCNTs. The sensingmaterial may be placed in between interdigitated electrodes of a sensor.When methane gas adsorbs to the sensing material, its electronic stateis changed resulting in a change in resistivity that is proportional tothe amount of methane adsorbed. This change in resistivity can bemeasured via a number of resistivity measurement techniques (e.g.,voltammetry).

In some embodiments, a process for fabrication of the SWCNT/NP materialsincludes an annealing step to remove oxygenated functional groups aswell as minimize the presence of a polymer coating of the metalnanoparticles. The annealing step may be performed at temperatures ofabout 350° C. To aid in removal of polymer material and functionalgroups, the annealing process may be performed in vacuum or in an inertenvironment such as nitrogen or argon gas. After annealing, theinterface between the SWCNT and the metal nanoparticles may include acarbonized or deoxygenated organic material.

In some embodiments, Platinum (Pt) or Palladium (Pd) nanoparticles areused due to their sensitivity to hydrocarbons including alkane gasessuch as methane, ethane and propane. When the nanoparticles are attachedto a conductive or semiconductive material such as a single-walledcarbon nanotube (SWCNT) conductivity changes associated with hydrocarbongas adsorption are measureable. For practical applications, it isimportant that the sensing materials be relatively insensitive tohumidity. Even in conventionally dry conditions, ambient concentrationsof water vapor are typically two to three orders of magnitude higherthan the concentrations of hydrocarbon gases being monitored. Forexample, the concentration of a hydrocarbon gas may be on the order of 1ppm or lower while water vapor concentrations are significantly higher.

In some embodiments, platinum-decorated SWCNT (Pt/SWCNT) materials aresynthesized by combining Pt particles and SWCNTs. The Pt nanoparticlesmay be synthesized by the so called polyol method that includesreduction of Pt salts in the presence of a polyol such as ethyleneglycol. During this process, Pt ions are reduced to Pt metal atoms whichform nanocrystals that are stabilized by a polymer that has goodmiscibility in ethylene glycol. For example, in some implementations,polyvinylpyrrolidone (PVP) may be used as a stabilizing polymer for themetal nanoparticles. Other examples of suitable polymers forstabilization of the Pt particles include polyethylene glycol (PEG),poly vinyl alcohol (PVA), poly methacrylic acid, dodecanethiol or othersubstituted analogues. The addition of the polymer may provide atemplate to obtain evenly distributed nanoparticles.

In alternate embodiments, platinum-decorated SWCNT (Pt/SWCNT) materialsare synthesized by formation of polymer coated platinum nanoparticlesdirectly onto the surface of the carbon nanotubes from metal saltsprecursors. In this embodiment, the Pt nanoparticles are stillsynthesized by reduction of Pt salts in ethylene glycol and a dispersantsuch as PVP, but in the presence of carbon nanotubes dispersed in thereaction mixture. The Pt nanoparticles growth is directly initiated ontothe surface of the carbon nanotubes.

The carbon nanotubes may be unfunctionalized so that they may besubstantially free of —COOH or —OH groups, or may have a degree ofcarboxylic acid (—COOH) or hydroxyl (—OH) functionalization. In someembodiments, a lower degree of functionalization or no functionalizationmay be beneficial for the sensor performance because it keepsundisturbed the electronic structure of the carbon nanotubes. In otherembodiments, a higher degree of —COOH an —OH functionalization may bealso beneficial because the carboxyl functional groups are negativelycharged and can donate their electrons, thus serving as a binding moietytowards Pt nanoparticles, increasing the surface coverage of the SWCNTsby the metal nanoparticles. In addition, the —COOH and —OH groups ontothe surface of oxidized SWCNT may enable the SWCNT to be dispersed in anaqueous solvent and ethylene glycol, which is highly desirable forfabrication of well dispersed Pt/SWCNT inks for sensor printing. Theoptimal degree of functionalization may depend on multiple factors: thetype of nanoparticle metal, the type of particle coating polymer and thegas to be sensed. The weight % of —COOH and —OH onto carbon nanotubes ismeasured by the weight loss in thermogravimetric analysis (TGA) attemperatures below 300° C. when the experiment is run in UHP grade inertgas. In some embodiments, the —COOH and —OH functional groups are beingcreated by extensively oxidizing SWCNT with nitric acid reflux beforecombining with the metal nanoparticles. A suitable weight % of —COOH and—OH oxygen containing groups ranges from zero (substantially free offunctional groups) to up to about 20%. In some embodiments, the rangemay be about 0.1% to about 5%, about 1% to about 5%, about 4% to about6%, or another range.

After generating the Pt/SWCNT material as described above, it may beannealed by heating it at a high temperature. For example, the materialmay be heated at 350° C. for a half hour. In various implementations,the heating may be performed at temperatures between about 350° C. andabout 500° C. Furthermore, the annealing process may be performed forabout half an hour, an hour, up to four hours, or at another length oftime. In some implementations, annealing may be performed in stages atdifferent temperatures and for different lengths of time. The annealingtemperature should be high enough to carbonize the polymer materialwhile low enough to prevent the material from catching fire or degradingdue to high temperatures. In some implementations, the annealing processmay be performed under vacuum conditions or in the presence of an inertgas to remove the hydrophilic oxygen containing groups from thePt/SWCNT.

While described generally with reference to Pt nanoparticles, in variousimplementations, the processes for synthesizing the Pt/SWCNT may beperformed similarly for other metal nanoparticles. For example, Pdnanoparticles may be used and synthesized into Pd/SWCNT using similarprocedures. Additional metal nanoparticles may be used for natural gassensing or for sensing of other gases. For example, in some embodiments,gold nanoparticles may be used for sensing of hydrogen sulfide. Othermetal nanoparticles may be used so long as their size doesn't cause themto burn during annealing processes or otherwise degrade during one ofthe processing steps. In other embodiments, one or more of palladium,iridium, rhodium, platinum, copper, nickel, chromium, ruthenium, silveror gold may also be used. In other embodiments, the metal decoratingnanoparticles consist of an alloy or a mixture of two or more metals.

During synthesis of the metal nanoparticles, PVP, or another polymer,may be used in large concentrations to encapsulate and stabilize the Ptnanoparticles. For example, in some implementations, theweight-to-weight (w/w) ratio by raw materials may be 10 mg SWCNT: 50 mgH2PtC16.6H2O: 250 mg PVP. However, as PVP may interfere with the sensorresponse as the PVP residue functions as a nonconductive material anddiffusion barrier some of the polymer material may be removed duringprocessing.

There are several choices of base SWCNT materials to use in thesynthesis. SWCNTs with no functional groups are advantaged as they arelargely insensitive towards humidity. However, they are difficult todisperse and do not bind well with the metal nanoparticles. Asmentioned, SWCNT with carboxyl functional groups (SWCNT-COOH) binds welland is easily dispersed in solvent. Nevertheless, SWCNT-COOH has thedisadvantage of being very sensitive to humidity. Therefore, after thePt is bound with the SWCNT surface, removal of carboxylated functionalgroup is important for enabling the sensors to function in real-worldconditions. We achieve this with heat treatment to induce thermaldegradation of PVP. In addition, the carboxyl groups of SWCNT-COOH aregradually degraded between 100 and 250° C. This can be seen as a 20%weight decrease up to 350° C. Degradation of the carboxylic acid groupsmay be the result of the decarboxylation of SWCNT-COOH.

Annealing of the Pt/SWCNT wherein the Pt particles are coated with PVPand the SWCNTs are functionalized with —COOH groups, may result in lossof PVP and —COOH groups. This may provide additional benefits comparedto non-annealed Pt/SWCNT. First, the annealed Pt/SWCNT may provideenhanced sensitivity because of lower polymer content and more directcontact between the Pt particles and the SWCNT surface. Second, theannealed Pt/SWCNT may provide reduced water sensitivity because ofremoval of the hydrophilic carboxyl and other oxygen containingfunctional groups. This renders the annealed Pt/SWCNT particles morehydrophobic when compared with the Pt/SWCNT particles that are notannealed.

In some implementations, a process for Pt/SWCNT synthesis begins withcombining about 1 mL of SWCNT-COOH solution (10 mg/mL H₂O) with about 10mL of Pt nanoparticles solution (65 mM). The combination may be mixedfor about 2 min. The solution may then be probe-sonicated for 5 min at30 W power. After mixing, the combination may be bath-sonicated for 1hour. The bath sonicated solution may then be placed in a convectionoven and EG was evaporated at about 200° C. After the solvent (e.g.,ethylene glycol) has evaporated, the solution may be moved into vacuumfurnace and heated to 350° C. for 30 min. To produce the sensors, thecombined nanoparticle and nanotube residue may be diluted with about 2mL of DMF and bath-sonicated briefly (<5 sec). The processes describedmay produce highly hydrophobic composite nanoparticles.

In some implementations, the contact angle measurement of the combinednanoparticles coated on glass slides shows significant change of thehydrophilic behavior between metal nanoparticle coated nanotubes beforeand after annealing. Coated Pt/SWCNT slides may have a low contact anglefor water (e.g., about 34 degrees), which demonstrates hydrophiliccharacteristics. After annealing, SWCNT-NP may have a much highercontact angle (e.g., about 83 degrees) which demonstrates a improvedhydrophobic characteristic when compared with the materials beforeannealing.

A sensing device as described herein may include a CNT/nanoparticlematerial deposited on interdigitated metal electrodes. In someembodiments, the electrodes consist of gold vacuum-deposited to about200 nm thick on a flexible polyethylene napthalate substrate. In otherembodiments, the electrodes are formed by printed silver nanoparticleinks. Other substrates and deposition methods may also be used togenerate a sensor as described herein. Etched or milled copper, such ason commercial printed circuit boards (PCBs) can also be used. In someimplementations, a fixed volume of ink containing SWCNT/NPs is printedon the channels of the sensing device. In some embodiments, the volumeof the ink dots (typically, 0.1-0.25 μL) and temperature of the heatedsurface (typically, 90° C.-110° C.) is determined by the resistancerequired for the sensing device for optimal performance.

FIG. 1 is a diagram of an example Pt/SWCNT sensing material 100. Thesensing material 100 may include a carbon nanotube 120 with attached Ptnanoparticles 110. As shown the Pt nanoparticles 110 are attacheddirectly to the carbon nanotube 120. This may provide a connection withno hydrophilic properties that has Pt nanoparticles 110 for detectingthe presence of natural gas. For example, the close connection betweenthe Pt nanoparticles 110 and the carbon nanotubes 120 may provide anaccurate indication of the concentration of natural gas such as methanedue to change in resistance of the combined Pt/SWCNT material 100.However, the combination of the Pt nanoparticles 110 and the carbonnanotubes 120 may not be realized without additional binders to combinethe materials.

FIG. 2A is a diagram of an example Pt/SWCNT sensing material 200 thatuses a carbon nanotube 220 with carboxyl functional groups (—COOH) 230combined with polymer coated metal nanoparticles 210. Carbon nanotube220 represents a carbon nanotube with functionalization with —COOH orother hydrophilic functional groups. The carboxyl functional may bepresent along the surface of the carbon nanotubes 220, as well as at theends of the carbon nanotubes. As shown in FIG. 2A, the carboxylfunctional groups 230 may aid in distribution and attachment of polymercoated nanoparticles 210 with carbon nanotubes 220. Accordingly, ratherthan a carbon nanotube 220 without carboxyl functional groups 230,polymer coated nanoparticles 210 may be combined with a carbon nanotube220 having carboxyl functional groups 230 to improve distribution andattraction of polymer coated nanoparticles 210 with the carbon nanotube220.

FIG. 2b is a diagram showing a configuration of a polymer coatednanoparticle 210 including a Pt nanoparticle 240 coated with a polymercoating 250. As described, the polymer coating 250 may comprise PVP oranother polymer coating to improve dispersion of the nanoparticles andattachment of the nanoparticles to nanotubes 220. The polymer coating250 may additionally reduce the hydrophilic properties of the polymercoated nanoparticle 210 compared to a non-coated nanoparticle such thatit enables adsorption of natural gas and limits adsorption of watervapor.

FIG. 3 is a diagram showing changes of a polymer coating 330 to apolymer coating 350 during an annealing process 345. As described above,a metal nanoparticle 310 attached to carbon nanotube 320 is depositedand coated. The metal nanoparticle 310 may be distributed through thecarbon nanotubes 320 based on the polymer coating 340. As shown in FIG.3, Pt nanoparticles are heat treated in an annealing process to removehydrophilic functional groups. Furthermore, certain portions of polymercoating 340 may be removed to generate a carbonized coating 350 as shownin FIG. 3.

FIG. 4 is a flow diagram illustrating a method 400 for producing metalnanoparticle-decorated nanotubes in accordance with embodiments of thepresent disclosure, by mixing polymer coated metal nanoparticles withcarbon nanotubes. The method 400 begins at block 402, where a firstsolution comprising polymer-coated metal nanoparticles is provided. Thepolymer-coated metal nanoparticles may have structures represented bythe polymer-coated metal nanoparticle 210 described with respect toFIGS. 1-3 above. The polymer-coated metal nanoparticles may besynthesized as described herein, or using any adaptations or othersuitable synthesis methods. In some embodiments, each polymer-coatedmetal nanoparticle comprises a Pt or Pd core and a polymer layercomprising PVP. In some embodiments, the polymer-coated metalnanoparticles are dispersed in an organic solvent.

At block 404, a second solution comprising carbon nanotubes is provided.The carbon nanotubes may have functional groups including carboxylicacid (—COOH) and/or hydroxyl (—OH) groups to improve attachment of metalnanoparticles to the carbon nanotubes. The carbon nanotubes may besynthesized as described herein, or using any other adaptations or othersuitable synthesis methods. In some embodiments, the carbon nanotubesare SWCNTs. In some embodiments, the carbon nanotubes may be dispersedin an aqueous solvent prior to combination with the metal nanoparticles.

At block 406, a reaction mixture is formed by combining the firstsolution with the second solution. The combined mixture causes the metalnanoparticles to deposit on the carbon nanotubes.

At block 407, the reaction mixture may be ultrasonicated. For example,the mixture may be ultrasonicated at high power for about 5 minutes orat low power for about an hour. In some embodiments, the mixture may beultrasonicated at high power for 5 minutes and low power for an hour.These processes might remove PVP from Pt nanoparticles temporarily,expose the Pt nanoparticle surface to SWNT, and enhance binding betweenSWCNT-COOH and Pt nanoparticles.

At block 408, the reaction mixture is heated to a temperature greaterthan a glass transition temperature of the polymer of the polymer-coatedmetal nanoparticles such as 200° C. (a glass transition temperature forPVP, for example, may vary from 100° C. to 180° C. depending on itsmolecular weight). This process also helps bindings of the nanoparticlestowards the SWNT surface. PVP residue might stay in EG or as outer shellof Pt/SWNT. In some embodiments, the nanoparticle-coated metalnanoparticles are treated with a solvent to remove solvent-accessiblePVP from surfaces of the nanoparticles.

At block 410, the dry PVP coated metal nanoparticle decorated carbonnanotubes are annealed. The annealing temperature may be at or aroundabout 350° C. In some implementations the annealing temperature may behigher or lower than described herein. The annealing process may furtherbe carried out for about half an hour. In some implementations, theannealing process may be performed for a longer or shorter amount oftime. In some embodiments, the resulting carbon nanotubes have a set ofattached metal nanoparticles with reduced polymer layers.

FIG. 5 is a flow diagram illustrating a method 500 for producing metalnanoparticle-decorated nanotubes in accordance with embodiments of thepresent disclosure, by direct formation of polymer coated metalnanoparticles onto the surface of the carbon nanotubes from metal saltsprecursors. The method 500 begins at block 502 where a first solutioncomprising metal salts and a polymer dispersant in a solvent isprovided. At block 504 a solution comprising carbon nanotubes dispersedin a solvent is provided. At block 506 a reaction mixture is formed bycombining the first solution with the second dispersion. At block 508the mixture is heated at a temperature to enable reduction of the metalsalts to metal atoms by a polyol such as ethylene glycol. For example,the mixture may be heated to a temperature of greater than 150° C. inthe case ethylene glycol is used. In some embodiments, the reactiontakes place in a vessel armed with a condenser to enable polyol solventreflux.

At block 510 the condenser may be removed and the mixture is heated at atemperature that is sufficient to remove the solvents. For exampleethylene glycol which boils at 197° C. can easily be removed buy heatingat a temperature of about 200° C. as described with reference to block408 above.

Finally, at block 512 the dry particles obtained after solventevaporation in block 510 may be annealed at a higher temperature. Theannealing temperature may be at or around about 350° C. In someimplementations the annealing temperature may be higher or lower thandescribed herein. The annealing process may further be carried out forabout half an hour. In some implementations, the annealing process maybe performed for a longer or shorter amount of time. In someimplementations, the annealing process may be performed for a length oftime until the solvents used to combine the metal nanoparticles andcarbon nanotubes is evaporated. In some embodiments, the resultingcarbon nanotubes have a set of attached metal nanoparticles with reducedpolymer layers.

FIGS. 6A-6C are diagrams depicting top down and cross-sectional views,respectively, of an exemplary sensor chip 600 according to an embodimentof the present disclosure. The sensor chip 600 includes a substrate 602having sensors 604A-604D formed on the surface of a semiconductorsubstrate. Although four sensors are depicted, fewer or additionalsensors may be used in various embodiments. The substrate 602 mayinclude a non-conductive material, such as polyethylene naphthalate(PEN), polyimide, or any other suitable non-conductive material. In someembodiments, a thickness of the substrate may be selected to facilitatea secure connection to a type of zero insertion force (ZIF) connector(e.g., substrate thickness of 250 μm).

The sensors 604A-604D may include any suitable electrode material suchas copper, graphite, titanium, silver, gold, platinum, or combinationsthereof. The sensors 604A-604D may further be shaped to facilitateelectrical contact with external components to provide improved output.A single counter electrode 606 may also be formed on the substrate 602,which may be shared by each of the sensors 604A-604D to reduce the totalnumber of electrodes on the sensor chip 600. In some embodiments,multiple return electrodes such as common electrode 606 may also be usedin a sensor.

The sensors 604A-604D and the common electrode 606 may together define aregion with interdigitated electrodes 608. FIG. 6B illustrates across-section through the interdigitated electrodes 608 of the sensors604A-604C, which shows an active region 610 defined between theinterdigitated electrodes 608 where a gas-sensing material may bedeposited. In certain embodiments, a thickness of the interdigitatedelectrodes 608 may range from 100 nm to 1 μm. A pitch 610 betweenadjacent interdigitated electrodes 608 may range, for example, from 50μm to 5 mm.

The sensor chip 600 may be designed such that a portion of the sensorchip 600 can be directly inserted into an electrical connector forresistance measurement. In order to achieve desired resistance levels ofthe printed substance, the sensor chip 600 can be designed to vary thenumber, duty cycle, and dimensions (including thickness) ofinterdigitated electrodes 608, as well as the gap distance betweenadjacent electrodes. The dimensions of the printed leads of the sensors604A-604D may be chosen such that the lead resistances for the commonelectrode path and the sensor path are nearly in order to cancel outtheir influence in the resistance measurements. The substrate 602 may bedesigned for ratiometric 3-wire resistance measurements, but may also becompatible with traditional 3-wire resistance measurements and 2-wireresistance measurements.

FIG. 6C illustrates the operation of the sensor chip 600, which isillustrated as being operatively coupled to a processing device 650having a number of pins 654. The processing device may include one ormore electronic components, such as a multiplexer 652, that may beconfigured to measure the resistivity of the gas sensing material usingthe various on-chip sensors. In some embodiments, the processing device650 represents one or more general-purpose processing devices such as amicroprocessor, central processing unit, or the like. For example, theprocessing device 650 may be a complex instruction set computing (CISC)microprocessor, reduced instruction set computing (RISC) microprocessor,very long instruction word (VLIW) microprocessor, or a processorimplementing other instruction sets or processors implementing acombination of instruction sets. The processing device 650 may also beone or more special-purpose processing devices such as an applicationspecific integrated circuit (ASIC), a field programmable gate array(FPGA), a digital signal processor (DSP), network processor, or thelike. The processing device 650 may be configured to perform variousoperations, such as applying electrical signals, measuring electricalsignals, and computing data based on the measured electrical signals. Incertain embodiments, the processing device 650 may be a single device ora device that controls other devices. For example, the processing device650 may be configured to perform resistivity measurements, or maycontrol one or more other devices that perform resistivity measurements.

FIG. 7 is a flow diagram illustrating a method 700 for fabricating asensor in accordance with embodiments of the present disclosure. Themethod 700 begins at block 702, where an ink comprising metalnanoparticle-decorated nanotubes is provided. The metalnanoparticle-decorated nanotubes may correspond to any metalnanoparticle-decorated nanotubes described herein (e.g., PVP-coatedpalladium nanoparticles bound to SWCNTs).

At block 704, a sensor chip or substrate having an electrode arrayformed thereon is provided. For example, a substrate and sensor asdescribed with reference to FIG. 6 may be provided. At block 706, theink is deposited between electrodes of the electrode array, and thesolvent of the ink is allowed to evaporate leaving a gas-sensingmaterial comprising metal nanoparticle-decorated nanotubes. For example,an ink may be provided having metal nanoparticle decorated nanotubes asdescribed above. In certain embodiments, the ink is deposited byprinting directly (e.g., using inkjet printing) on the sensors. In someembodiments, the ink is deposited using other suitable methods, such aspipetting, spin-coating, or dip-coating.

At block 708, the sensor may be heated of about a temperature of about100° C. to evaporate the solvent. In some embodiments, the sensor may beheated at a higher or lower temperature to evaporate the solvent.

In some embodiments, a sensor produced according to the method 700 mayoperate over a wide relative humidity range in ambient conditions. Asused herein, “ambient conditions” refers to the conditions of a typicallaboratory environment with a temperature of 20±5° C. and a pressure of1±0.1 atmospheres (ATM). In certain embodiments, during operation of thesensor in an ambient environment having a relative humidity from 0% to80%, the sensor has a methane detection limit of 100 ppm. In someembodiments, the sensor may have a lower detection limit, e.g., 50 ppmor 10 ppm. The sensor may achieve such performance over a temperaturerange outside of the ambient conditions (e.g., from −5° to 50° C.).

Printed methane sensors based on hydrophobic sensing particles sufferfrom reduced performance in high and prolonged humidity environment.Disclosed herein are structures and processes for fabrication of highlyhydrophobic polymer-coated metal nanoparticle decorated CNTs with afinal annealing step at >350 deg. C, in vacuum or inert gas. The methodincreases the contact between the SWCNT surface and the metalnanoparticles. This produces sensors with improved sensitivity anddetection limits and with increased lifetime in humid conditions.

Various operations are described as multiple discrete operations, inturn, in a manner that is most helpful in understanding the presentdisclosure, however, the order of description may not be construed toimply that these operations are necessarily order dependent. Inparticular, these operations need not be performed in the order ofpresentation.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of several embodiments of thepresent disclosure. It will be apparent to one skilled in the art,however, that at least some embodiments of the present disclosure may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram format in order to avoid unnecessarily obscuringthe present disclosure. Thus, the specific details set forth are merelyexemplary. Particular embodiments may vary from these exemplary detailsand still be contemplated to be within the scope of the presentdisclosure.

Additionally, some embodiments may be practiced in distributed computingenvironments where the machine-readable medium is stored on and orexecuted by more than one computer system. In addition, the informationtransferred between computer systems may either be pulled or pushedacross the communication medium connecting the computer systems.

Embodiments of the claimed subject matter include, but are not limitedto, various operations described herein. These operations may beperformed by hardware components, software, firmware, or a combinationthereof.

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be in an intermittent oralternating manner.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The words “example” or“exemplary” are used herein to mean serving as an example, instance, orillustration. Any aspect or design described herein as “example” or“exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Rather, use of the words“example” or “exemplary” is intended to present concepts in a concretefashion. As used in this application, the term “or” is intended to meanan inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both A and B, then “X includesA or B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Moreover, use of the term “an embodiment” or “one embodiment” or“an implementation” or “one implementation” throughout is not intendedto mean the same embodiment or implementation unless described as such.Furthermore, the terms “first,” “second,” “third,” “fourth,” etc. asused herein are meant as labels to distinguish among different elementsand may not necessarily have an ordinal meaning according to theirnumerical designation.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.The claims may encompass embodiments in hardware, software, or acombination thereof.

What is claimed is:
 1. A method comprising: providing carbon nanotubesdecorated with polymer coated metal nanoparticles; and annealing themetal coated carbon nanotubes to reduce a quantity of hydrophiliccomponents of the polymer coating.
 2. The method of claim 1, wherein themetal nanoparticles comprise a metal selected from a group comprisingpalladium, iridium, rhodium, platinum, copper, nickel, chromium,ruthenium, silver and gold.
 3. The method of claim 2 wherein the metalnanoparticles comprise an alloy of two or more of the metals selectedfrom a group comprising palladium, iridium, rhodium, platinum, copper,nickel, chromium, ruthenium, silver and gold.
 4. The method of claim 1wherein the provided carbon nanotubes contain up to 5% oxygenatedfunctional groups.
 5. The method of claim 1, wherein the carbonnanotubes comprise single-walled carbon nanotubes.
 6. The method ofclaim 1, wherein the polymer layer comprises polyvinylpyrrolidone, andwherein the metal nanoparticles comprise palladium or platinum.
 7. Themethod of claim 1,wherein the annealed metal coated carbon nanotubes arehydrophobic.
 8. The method of claim 1, wherein annealing the metalcoated carbon nanotubes is performed at above 350° C.
 9. The method fromclaim 1, further comprising producing the carbon nanotubes decoratedwith polymer coated metal nanoparticles by: providing carbon nanotubes;and mixing polymer coated metal nanoparticles with the carbon nanotubesto generate metal coated carbon nanotubes.
 10. The method from claim 1,further comprising synthesizing the carbon nanotubes decorated withpolymer coated metal nanoparticles by direct formation of polymer coatedmetal nanoparticles onto the surface of the carbon nanotubes from one ormore metal salts precursors.
 11. The method of claim 1, whereinannealing the metal coated carbon nanotubes is performed in a vacuum orinert gas environment.
 12. A composition of matter comprising: carbonnanotubes combined with polymer coated metal nanoparticles, wherein thecarbon nanotube and polymer coated metal nanoparticles have beenannealed to remove oxygenated functional groups.
 13. The composition ofmatter of claim 12, wherein the metal nanoparticles comprise a metalselected from a group consisting of palladium, iridium, rhodium,platinum, copper, nickel, chromium, ruthenium, silver and gold.
 14. Thecomposition of matter of claim 12, wherein the carbon nanotubes comprisesingle-walled carbon nanotubes.
 15. The composition of matter of claim12, wherein the polymer layer comprises polyvinylpyrrolidone, andwherein the metal nanoparticles comprise palladium or platinum.
 16. Thecomposition of matter of claim 12, wherein the annealed metal coatedcarbon nanotubes are hydrophobic.
 17. The composition of matter of claim12, wherein annealing is performed at above 350° C.
 18. The compositionof matter of claim 12, wherein oxygenated functional groups are removedfrom the polymer coated metal nanoparticles and the single walled carbonnanotubes during the annealing.
 19. The composition of matter of claim12, wherein residual polymer content is less than 50% after annealing20. A sensor for detecting gas, the sensor comprising: an electrodeassembly comprising electrodes; and a gas-adsorbing material disposedbetween the electrodes of the electrode assembly, wherein thegas-adsorbing material comprises: carbon nanotubes; and polymer-coatedmetal nanoparticles bound to the carbon nanotubes, wherein the polymercoated metal nanoparticles are annealed to reduce an amount of polymeron the sensor.